Observations of Structural Damage to Girder Bridges in Iraq Caused by Blast Threats

This paper summarizes the blast events and resulting damage found on more than 100 bridges in Iraq caused by terrorist attacks. Most structures were two to three lanes wide and composed of a superstructure with prestressed concrete beams that supported a concrete deck. In most cases, terrorists placed charges either on the deck or at the base of columns. Typical structural system failures were due to either intermediate column failure, girder end failure, or girder end failure in combination with midspan failure. It was found that a single blast load on the deck rarely caused collapse, while three or more blast loads distributed across the deck nearly always caused deck and/or span collapse. If at least two girders remained largely undamaged, one lane of traffic could often be directed safely over the structure, allowing a heavily damaged bridge to remain in temporary use. When columns were attacked, charges placed symmetrically caused the columns to collapse downward, whereas charges placed asymmetrically caused significant lateral movement. Although most bridges were simple span, substantial resiliency was observed, where the structures could support large amounts of load even when some critical components theoretically had near-zero capacity. It was further found that separating components, even slightly, could significantly reduce blast damage propagation. Brief recommendations are given to mitigate blast damage.